High density semiconductor memory cell and memory array using a single transistor and having variable gate oxide breakdown

ABSTRACT

A programmable memory cell comprised of a transistor located at the crosspoint of a column bitline and a row wordline is disclosed. The transistor has its gate formed from the column bitline and its source connected to the row wordline. The memory cell is programmed by applying a voltage potential between the column bitline and the row wordline to produce a programmed n+ region in the substrate underlying the gate of the transistor. Further, a gate dielectric of the transistor has a higher breakdown voltage near the source connected to the row wordline than its drain.

RELATED APPLICATIONS

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 10/677,613 filed Oct. 2, 2003 entitled “HIGHDENSITY SEMICONDUCTOR MEMORY CELL AND MEMORY ARRAY USING A SINGLETRANSISTOR HAVING A BURIED N+ CONNECTION”, which is acontinuation-in-part of U.S. patent application Ser. No. 10/448,505filed May 30, 2003, now U.S. Pat. No. 6,856,540 entitled “HIGH DENSITYSEMICONDUCTOR MEMORY CELL AND MEMORY ARRAY USING A SINGLE TRANSISTOR”and U.S. patent application Ser. No. 10/133,704 filed Apr. 26, 2002, nowU.S. Pat. No. 6,777,757 entitled “HIGH DENSITY SEMICONDUCTOR MEMORY CELLAND MEMORY ARRAY USING A SINGLE TRANSISTOR”, to which priority from allis hereby claimed under 35 USC § 120.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a nonvolatile programmablesemiconductor memory, and more particularly to a single transistormemory cell programmed by break down of the transistor gate oxide, and amemory array incorporating such cells.

BACKGROUND OF THE INVENTION

Nonvolatile memory retains stored data when power is removed, which isdesirable in many different types of electronic devices. One commonlyavailable type of nonvolatile memory is the programmable read-onlymemory (“PROM”), which uses word line—bit line crosspoint elements suchas fuses, anti-fuses, and trapped charge devices such as the floatinggate avalanche injection metal oxide semiconductor (“FAMOS”) transistorto store logical information.

An example of one type of PROM cell that uses the breakdown of a silicondioxide layer in a capacitor to store digital data is disclosed in U.S.Pat. No. 6,215,140 to Reisinger et al. The basic PROM disclosed byReisinger et al. uses a series combination of an oxide capacitor and ajunction diode as the crosspoint element (the term “crosspoint” refersto the intersection of a bitline and a wordline). An intact capacitorrepresents the logic value 0, and an electrically broken-down capacitorrepresents the logic value 1. The thickness of the silicon dioxide layeris adjusted to obtain the desired operation specifications. Silicondioxide has a breakdown charge of about 10 C/cm² (Coulomb/cm²). If avoltage of 10 volts is applied to a capacitor dielectric with athickness of 10 nm (resultant field strength 10 mV/cm), a current ofabout 1 mA/cm² flows. With 10 volts, this thus results in a substantialamount of time for programming a memory cell. However, it is moreadvantageous to design the capacitor dielectric to be thinner, in orderto reduce the high power loss which occurs during electrical breakdown.For example, a memory cell configuration having a capacitor dielectricwith a thickness of 3 to 4 nm can be operated at about 1.5 V. Thecapacitor dielectric does not yet break down at this voltage, so that1.5 V is sufficient to read data from the memory cell. Data are stored,for example, at 5 V, in which case one cell strand in a memory cellconfiguration can be programmed within about 1 millisecond. The energyloss which occurs in this case per cm² of capacitor dielectric is thenabout 50 Watts (10 Coulomb*5 V). If the desired power loss is about 0.5W, about 100 seconds are required to program a 1 Gigabit memory. If thepermissible power losses are higher, the programming can be carried outcorrespondingly more quickly.

Some types of nonvolatile memory are capable of being repeatedlyprogrammed and erased, including erasable programmable read onlysemiconductor memory generally known as EPROM, and electrically erasableprogrammable read only semiconductor memory generally known as EEPROM.EPROM memory is erased by application of ultraviolet light andprogrammed by application of various voltages, while EEPROM memory isboth erased and programmed by application of various voltages. EPROMsand EEPROMs have suitable structures, generally known as floating gates,that are charged or discharged in accordance with data to be storedthereon. The charge on the floating gate establishes the thresholdvoltage, or V_(T), of the device, which is sensed when the memory isread to determine the data stored therein. Typically, efforts are madeto minimize gate oxide stress in these types of memory cells.

A device known as a metal nitride oxide silicon (“MNOS”) device has achannel located in silicon between a source and drain and overlain by agate structure that includes a silicon dioxide layer, a silicon nitridelayer, and an aluminum layer. The MNOS device is switchable between twothreshold voltage states V_(TH(high)) and V_(TH(low)) by applyingsuitable voltage pulses to the gate, which causes electrons to betrapped in the oxide-nitride gate (V_(TH(high))) or driven out of theoxide-nitride gate (V_(TH(low))). Again, efforts are made to minimizegate oxide stress in these types of memory cells.

A junction breakdown memory cell that uses a stored charge on the gateof a gate controlled diode to store logic 0 and 1 values is disclosed inU.S. Pat. No. 4,037,243 to Hoffman et al. Charge is stored on the gateby using a capacitance formed between the p-type electrode of the gatecontrolled diode and the gate electrode. Charge storage is enhanced byusing a composite dielectric in the capacitor formed from silicondioxide and silicon nitride layers in place of silicon dioxide. Theapplication of an erase voltage to the electrode of the gate controlleddiode causes the oxide-nitride interface surface to fill with negativecharge, which is retained after the erase operation is completed. Thisnegative interface charge causes the gate controlled diode to operate inan induced junction mode even after the erase voltage is removed. Whenthe gate controlled diode is thereafter read, it exhibits field-inducedjunction breakdown of its channel and a saturation current flows. Thefield induced junction breakdown voltage is less than the metallurgicaljunction breakdown voltage. However, the application of a write voltageto the electrode of the gate controlled diode causes the silicondioxide/silicon nitride interface to fill with positive charge, which isretained after the write operation is completed. When the gatecontrolled diode is thereafter read, it will not break down because nochannel exists. Only a slight current flows. The different current flowsare sensed and indicate different logic states.

Improvements in the various processes used for fabricating the varioustypes of nonvolatile memory tend to lag improvements in widely usedprocesses such as the advanced CMOS logic process. For example,processes for devices such as flash EEPROM devices tend to use 30% moremask steps than the standard advanced CMOS logic process to produce thevarious special regions and structures required for the high voltagegeneration circuits, the triple well, the floating gate, the ONO layers,and the special source and drain junctions typically found in suchdevices. Accordingly, processes for flash devices tend to be one or twogenerations behind the standard advanced CMOS logic process and about30% more expensive on a cost-per-wafer basis. As another example,processes for antifuses must be suitable for fabricating variousantifuse structures and high voltage circuits, and so also tend to beabout one generation behind the standard advanced CMOS process.

Generally, great care is taken in the fabrication of the silicon dioxidelayer used in metal-oxide-silicon (MOS) devices such as capacitors andtransistors. The high degree of care is necessary to ensure that thesilicon dioxide layer is not stressed during manufacture or subsequentnormal operation of the integrated circuit, so that the desired devicecharacteristics are attained and are stable over time. One example ofhow much care is taken during fabrication is disclosed in U.S. Pat. No.5,241,200 to Kuroda, which discloses the use of a diffused layer and ashunt to discharge charges accumulated in the word line during a waferfabrication process. Avoiding this charge accumulation ensures that alarge electric field is not applied to the gate insulating film, so thatvariations in the characteristics of transistors using the word line astheir gate wiring line and degradation and breakdown of the gateinsulating film are prevented.

An example of how much care is taken in circuit design to avoidstressing the silicon dioxide layer of a transistor during normalcircuit operation is disclosed in U.S. Pat. No. 6,249,472 to Tamura etal. Tamura et al. disclose an antifuse circuit having an antifuse inseries with a p-channel MOS transistor in one embodiment and in serieswith an n-channel MOS transistor in another embodiment. While theantifuse is fabricated without the additional film manufacturingprocesses typically required for fabricating antifuse circuits, Tamuraet al. poses another problem. When the antifuse is shorted out, theseries-connected transistor is exposed to a high voltage sufficient tobreak down the silicon dioxide layer of the transistor. Tamura et al.disclose the addition of another transistor to the circuit to avoidexposing the first transistor to the break down potential.

The observations above generally indicate that there are stilldisadvantages with each of the prior art memory technologies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of a portion of a memory array inaccordance with the present invention.

FIG. 2 is a partial layout diagram of a portion of the memory arrayrepresented by FIG. 1.

FIG. 3 is a cross-section diagram of an integrated circuit structure forthe portion of the memory array corresponding to FIG. 2.

FIG. 4 is a table of voltages showing the operation of the memory cellof FIGS. 1-3.

FIG. 5 is a cross-section diagram of a memory cell that has beenprogrammed.

FIG. 6 is a schematic circuit diagram of a memory cell that has beenprogrammed.

FIG. 7 is a cross-section diagram of an experimental setup.

FIG. 8 is a graph showing the effect of a constant voltage stress on anultra-thin gate oxide.

FIG. 9 is a graph showing various stages in the current-voltagecharacteristics of an ultra-thin gate oxide as degradation proceeds.

FIG. 10 is a graph showing time-to-breakdown at 63% distribution vs.gate voltage in a semi-log scale measured on n-channel field effecttransistors (inversion) for various oxide thickness.

FIG. 11 is a graph showing the current-voltage characteristics of n-typedevices measured after the detection of successive breakdown events.

FIG. 12 is a is a partial layout diagram of a portion of the memoryarray formed in accordance with an alternative embodiment of the presentinvention.

FIG. 13 is a cross-section diagram of an integrated circuit structurefor the portion of the memory array corresponding to FIG. 12 taken alongline A-A′.

FIG. 14 is a cross-section diagram of an integrated circuit structurefor the portion of the memory array corresponding to FIG. 12 taken alongline B-B′.

FIG. 15 is a table of voltages showing the operation of the memory cellof FIGS. 12-14.

FIG. 16 is a cross section view of one embodiment of memory cells formedin accordance with the present invention.

FIG. 17 is a schematic circuit diagram of the memory cells of FIG. 16.

FIG. 18 is a table of voltage showing the operation of the memory cellsof FIG. 16.

FIG. 19 is a top layout view showing the extent of a nitrogen implant inone method of forming the memory cells of FIG. 16.

FIGS. 20-23 show cross sectional views of one method for forming thememory cells of FIG. 16.

FIGS. 24-25 show cross sectional views of an alternative method forforming the memory cells of FIG. 16.

FIGS. 26-27 show cross sectional views of an alternative method forforming memory cells of the present invention.

DETAILED DESCRIPTION

A semiconductor memory cell having a data storage element constructedaround a gate oxide is used to store information by stressing theultra-thin dielectric into breakdown (soft or hard breakdown) to set aleakage current level of the memory cell. The memory cell is read bysensing the current drawn by the cell. A suitable ultra-thin dielectricis the high quality gate oxide of about 10-50 Å thickness or less usedin a transistor, as is commonly available from presently availableadvanced CMOS logic processes. Such oxides are commonly formed bydeposition, by oxide growth from a silicon active region, or by somecombination thereof. Other suitable dielectrics includeoxide-nitride-oxide composites, compound oxides, and so forth.

In the following description, numerous specific details are provided toprovide a thorough understanding of embodiments of the invention. Oneskilled in the relevant art will recognize, however, that the inventioncan be practiced without one or more of the specific details, or withother methods, components, materials, etc. In other instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring aspects of the invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

The present invention is related to other types of gate oxide breakdownbased non-volatile memory designs developed by the present inventor andassigned to the same assignee as the present invention. Examples areshown in U.S. patent application Ser. No. 09/955,641 filed on Sep. 18,2001 entitled “SEMICONDUCTOR MEMORY CELL AND MEMORY ARRAY USING ABREAKDOWN PHENOMENA IN AN ULTRA-THIN DIELECTRIC”, U.S. patentapplication Ser. No. 10/024,327 filed on Dec. 17, 2001 entitled“SEMICONDUCTOR MEMORY CELL AND MEMORY ARRAY USING A BREAKDOWN PHENOMENAIN AN ULTRA-THIN DIELECTRIC”, U.S. patent application Ser. No.09/982,034 filed on Oct. 17, 2001 entitled “SMART CARD HAVINGNON-VOLATILE MEMORY FORMED FROM LOGIC PROCESS”, and U.S. patentapplication Ser. No. 09/982,314 filed on Oct. 17, 2001 entitled“REPROGRAMMABLE NON-VOLATILE OXIDE MEMORY FORMED FROM LOGIC PROCESS”,each of which is hereby incorporated by reference. However, in each ofthe memory cells described above, the cell size is relatively large. Thepresent invention provides a much smaller cell size, thereby allowing ahigher density.

FIG. 1 shows an example of a memory array 100 formed in accordance ofwith the present invention. The memory array 100 is a four column bythree row array, however, it can be appreciated that the array can beany arbitrary size. The memory array 100 includes twelve memory cells102, each of which comprises a MOS transistor 104. The memory cell 102at, for example, the crosspoint of the first row R₁ and the first columnC₁ includes a MOS transistor 104 having its gate connected to a columnline C₁ (also referred to herein as a “bitline” or “column bitline”),its source connected to a row line R₁ (also referred to herein as a“wordline” or “row wordline”), and its drain left floating connected toa drain of an adjacent memory cell 102. Alternatively, as will be seenbelow, since there is no current through the drains, the drains ofadjacent devices need not be connected where a shallow trench isolation(STI) is used to isolate two memory cells

As will be seen below, during the programming step, a relatively largevoltage is applied to the gate of the transistor 102 (through thebitline C_(x), where x=1 to M and M is the total number of columns) ofthe selected column to break down the gate oxide of the transistor 102.The other memory cells 102 shown in FIG. 1 are also formed from, in oneembodiment, identical transistors 102 at the crosspoints of the columnbitlines C_(x) and row wordlines R_(y), where y=1 to N and N is thetotal number of columns.

The use of transistors 102 as data storage elements in the memory array100 of FIG. 1 is advantageous because the transistors can be fabricatedusing many conventional CMOS processes using only a single polysilicondeposition step, without adding any mask steps to them. This is incontrast to “floating gate” type flash memory, which requires at leasttwo polysilicon layers. Further, with modern technological advances, thesize of a transistor can be made very small. For example, current 0.18micron, 0.13 micron, and smaller line width processes would greatlyincrease the density of the flash memory.

Although only a four by three memory array 100 is shown, in practicesuch memory arrays contain on the order of about one gigabit or more ofmemory cells when fabricated using, for example, an advanced 0.13 μmCMOS logic process. Even larger memories will be realized as CMOS logicprocesses improve further. The memory array 100 in practice is organizedinto bytes and pages and redundant rows (not shown), which may be donein any desired manner. Many suitable memory organizations are well knownin the art.

FIG. 2 shows a partial layout diagram 200 for a portion of the memoryarray 100, and FIG. 3 presents a cross-section of an illustrative MOSintegrated circuit 300 showing the principal structural aspects thereofcorresponding to the memory cells 102 formed by transistors 104 inaccordance with the layout diagram of FIG. 2. The layout diagram of FIG.2 is suitable for an advanced CMOS logic process. The term MOS iscommonly understood to pertain to any gate material, including dopedpolysilicon and other good conductors, as well as to various differenttypes of gate dielectrics not limited to silicon dioxide, and the termis so used herein. For example, the dielectric may be any type ofdielectric, such as an oxide or nitride, which undergoes a hard or softbreakdown upon the application of a voltage for a period of time. In oneembodiment, a thermally grown gate silicon oxide of about 50 angstroms(50 A for 0.25 μm, 30 A for 0.18 μm, 20 A for 0.13 μm, 16 A for 0.09 μmprocess) thick is used.

The memory array 100 preferably is laid out in a grid in which thecolumn lines such as C₁, C₂, C₃, and C₄ are orthogonal to the row linessuch as R₁, R₂, and R₃, as well as the diffused source and drains of thetransistors 104. The transistor 104 at the crosspoint of the row line R₁and the column line C₁ is formed in a p-well active region 302 in thefollowing manner.

An ultra-thin gate oxide layer 304 is formed by either deposition orthermal oxidation. This is followed by a deposition and doping of apolysilicon layer, which is patterned using a gate mask containingpatterns for the column bitlines C₁, C₂, C₃, and C₄, which also serve asgates 310 for the transistors 104. Alternatively, the column bitlinesmay be separate structure that are connected to the gates 310 of thetransistors through column bitline segments. The various source anddrain regions are formed by conventional process steps (implants,spacers, and n+ source/drain implants), creating the n+ source regions306 and n+ drain regions 308. Importantly, it should be noted that thepolysilicon gate 310 for the transistors 104 should not overlap with then+ source/drain regions. Thus, a lightly doped drain structure is notemployed. As will be seen below, by not having the polysilicon gate 310overlap or proximal to the n+ source/drain regions, during programming,the polysilicon gate will not short directly to the n+ source/drainregions.

Further, contacts (also referred to as row wordline segments) to n+source regions 306 are formed to allow connection with the row linesR_(y). The row lines R_(y) are formed from a metal deposition that issubsequently etched. Further, an interlayer dielectric (not shown) isdeposited over the polysilicon layer. Thus, the contact vias connectingthe metal row lines R_(y) to the n+ source regions 306 are formed withinthe interlayer dielectric.

The operation of the memory array 100 is now explained with reference tothe illustrative voltages shown in FIG. 4. It will be appreciated thatthe voltages are illustrative, and that different voltages are likely tobe used in different applications or when different process technologiesare used. During programming, the various memory cells in the memoryarray 100 are exposed to one of four possible programming voltagecombinations, which are shown on lines 401, 403, 405, and 407 of FIG. 4.Read voltages are shown on lines 409, 411, 413, and 415. Assume that amemory cell 102 is selected for programming and is located at thecrosspoint of R₁ and C₁. The selected memory cell 102 is referred to asat the selected row and selected column (“SR/SC”). As shown on line 401,the voltage on the selected wordline R₁ (designated as V_(wordline) or“voltage on the wordline”) is 0 volts and the voltage on the bitline C₁(designated as V_(bitline) or “voltage on the bitline”) is a programmingvoltage (V_(PP)), in this case 8 volts. Thus, the voltage across thegate (the bitline C₁) of the transistor 104 and the source (the wordlineR₁) of the transistor 104 is 8 volts. The gate oxide 304 of thetransistor 104 is designed to break down at this potential difference,which programs the memory cell. During programming, the voltagepotential breaks down the gate oxide and results in a leakage currentthrough the gate oxide into the underlying substrate and collectedmostly by the N+ source/drain, which is connected to ground. Further,the result of this is that a programmed n+ region 501 (see FIG. 5) isformed in the p-well 302 between the n+ source region 306 and the n+drain region 308 of the transistor 104.

It can be appreciated that the precise magnitude of voltages applied isdependent upon the thickness of the gate oxide and other factors. Thus,for example, for a 0.13 micron CMOS process, the gate oxide is typicallythinner, thereby requiring a lower voltage differential between theselected wordline and the selected bitline. In one embodiment, where a0.13 micron CMOS process is used, the bitline C₁, and the unselectedword line has a voltage of 4.5 volts and the unselected bitline R₁ has avoltage of between 0 and 1.2 volts.

With R₁ and C₁ being the selected row and column, consider the impact onthe memory cell 102 at the crosspoint of a selected row and anunselected column (“SR/UC”), e.g., R₁ and C₂. As shown on line 405, thevoltage on the wordline R₁ is 0 volts and the voltage on the unselectedbitline C₂ is 0 or floating. This causes a relatively low potentialdifference across the gate oxide 304 of the transistor 104, which isinsufficient to break down the gate oxide for the transistor 104 at thecrosspoint. The memory cell 102 does not program under these conditions.

With R₁ and C₁ being the selected row and column, consider the impact onthe memory cell 102 at the crosspoint of a selected column and anunselected row (“UR/SC”), e.g. R₂ and C₁. As shown on line 403, thevoltage on the unselected wordline R₂ is floating or V_(PP) and thevoltage on the bit line C₁ is at V_(PP) (8 volts in this example). Thiscauses a relatively low potential difference across the gate oxide 304of the transistor 104. The memory cell 102 does not program under theseconditions.

With R₁ and C₁ being the selected row and column, consider the impact onthe memory cell 102 at the crosspoint of an unselected column and anunselected row (“UR/UC”), e.g. R₂ and C₂. As shown on line 407, thevoltage on the unselected wordline R₂ is floating or V_(PP) and thevoltage on the unselected bitline C₂ is 0 volts or floating. This causesa negative potential difference across the gate 304 and the N+source/drain of the transistor 104. Since the N+ source/drain ispositive and gate is negative, the higher voltage on the source/drainwill not pass under the gate, so the memory cell 102 does not programunder these conditions. Further, the voltage on the unselected wordlinecould be biased to an intermediate voltage, such as 2V to 6V to preventthe cell from being programmed. However, the programmed cell will causea leakage current from the selected bitline to the unselected wordline.If the unselected bit line is floating, the leakage current will chargeit up, which causes the voltage to raise up in the bitline. By biasingthe unselected wordlines R_(x) to V_(PP), we can prevent this leakageand thus the charge time of the selected bitline through programmedcells can be reduced.

After a memory cell 102 has been programmed by breaking down the gateoxide 304, the physical characteristics of the cell 102 is changed.Turning to FIG. 5, a transistor 104 of the memory cell 102 has beenprogrammed. During programming, a programmed n+ region 501 is formedunderneath the gate of the transistor 104. This programmed n+ region 501is formed as current (during the programming process) pierces throughthe gate oxide 304 and deposits into the substrate (p-well 302).

Although difficult to view clearly in FIG. 3, as noted above, thepolysilicon gate 310 for the transistors 104 should not overlapvertically with the n+ source/drain regions 306 and 308. Indeed, theseparation laterally between the gate 310 and the n+ source region 306and n+ drain region 308, by for example the use of a CMOS LDD spacer,should be sufficient to prevent short circuiting during programming. Asseen in FIG. 3, this lateral separation is designated as a lateraldistance D. In one embodiment, the lateral distance D is between 0.02microns to 0.08 microns as formatted by a LDD dielectric spacer in CMOSlogic devices. By not having the polysilicon gate overlap or proximal tothe n+ source/drain regions, during programming, the polysilicon gatewill not short directly to the n+ source/drain regions. Instead, theprogrammed n+ region 501 is formed. Further, alternative methods bywhich short circuiting is avoided between the gate 310 and the n+regions 306 and 308 may be utilized. As but one example, the gate oxidenear the n+ regions 306 and 308 may be made thicker by poly gatesidewall oxidation after the gate poly etch. It can be appreciated thatother methods are also suitable.

The programmed memory cell of FIG. 5 can be seen in schematic form inFIG. 6. The result of programming a memory cell is the formation of twogated diodes 601 and 603. The gated diodes 601 and 603 prevent currentfrom flowing from the wordlines R_(y) to the bitlines C_(x). However,current will be allowed to flow from the bitlines C_(x) to the wordlinesR_(y) during the read operation since the positive gate bias can inducea n+ inversion, which can make the connection to the N+ source/drainregions.

The memory array 100 is read in the following manner. A read selectvoltage VRD (for example 1.8 volts) is placed on the selected columnbitline (“SC”) and a read select voltage of 0 volts is placed on theselected row wordline (“SR”). Note that these voltages are for a typical0.18 micron CMOS process. Lower voltages would be typically used forsmaller more advanced CMOS processes. For example, for a 0.13 micronCMOS process, the read select voltage on the selected column bitline maybe approximately 1.2 volts.

Assume that R₁ and C₁ are the selected row and column (“SC/SR”) and thatthe memory cell 102 at that crosspoint is programmed. As shown on line409, 1.8 volts (a read select voltage) is applied via bitline C₁ to thegate of the transistor 104 and 0 volts is applied to the source via thewordline R₁. This causes a current to flow from the bitline C₁, throughthe gate oxide of the transistor 104, and out through the wordline R₁,which is ground to zero. By detecting the current on the bitline, it canbe determined whether the memory cell 102 is programmed. If the memorycell 102 is not programmed, no current will flow, which indicates thatthe memory cell is not programmed.

With R₁ and C₁ being the selected row and column for the read operation,consider the impact on the memory cell 102 at the crosspoint of aselected column and an unselected row (“UR/SC”), e.g. R₂ and C₁. Asshown on line 411, 1.8 volts is on the selected bitline C₁ and thesource via the unselected wordline R₂ is held to floating or V_(RD).There is no voltage potential across the transistor and no current flow,which indicates that the memory cell is not programmed. By biasing theunselected wordline R₂ to V_(RD), the charge time of the selectedbitline through programmed cells can be reduced. This is because if theunselected wordline is floating, it will take some time to charge it upto by the selected bit through the programmed cell.

With R₁ and C₁ being the selected row and column for the read operation,consider the impact on the memory cell 102 at the crosspoint of anunselected column and a selected row (“SR/UC”), e.g. R₁ and C₂. As shownon line 413, 0 volts is on the unselected bitline C₂ and 0 volts isapplied to the source via the selected wordline R₁. There is no voltagepotential across the transistor and no current flow, which indicatesthat the memory cell is not programmed.

With R₁ and C₁ being the selected row and column for the read operation,consider the impact on the memory cell 102 at the crosspoint of anunselected column and an unselected row (“UR/UC”), e.g. R₂ and C₂. Asshown on line 415, 0 volts is on the unselected bitline C₂ and thesource via the unselected wordline R₂ is held to floating or V_(RD).Even for the cell that was previously programmed, and the programmedcell acts like a reversed biased diode, so there is no current goingfrom the unselected wordline (1.8V) to the unselected bitline (0V),which indicates that the memory cell is not programmed.

Thus, as seen above, during the read cycle, no current is drawn bymemory cells at crosspoints having either an unselected row or anunselected column. Note that the unselected wordlines may be leftfloating. This embodiment will tend to reduce leakage current throughthe wordlines, as well as allowing the use of smaller word line drivers,thereby saving integrated circuit space.

Moreover, in an alternative embodiment, in order to increase the n+source/drain junction breakdown voltage and reduce junction leakage inthe case of an unselected wordline charged to V_(PP) either from theselected word line through prior programmed cells or through a word linedriver, a high energy, low dose n+ implant can be used. The implant maybe a standard n+ electrostatic discharge protection implant from aconventional CMOS process or other existing implant steps, thus stayingwithin standard CMOS logic process. Still, in other embodiments, aspecial implant step may be added to optimize the implanting.

An alternative embodiment of the present invention is shown in FIGS.12-14. The operation tabitlinee of this alternative embodiment is shownin FIG. 15. In the alternative embodiment of FIG. 12, the row wordlinesR₁ and R₂ are not formed from a metal deposition as is shown in theembodiment of FIG. 2. Instead, the row wordlines R₁ and R₂, and ingeneral all of the row wordlines R_(Y), are formed from a buried n⁺layer formed in the substrate. The buried n+ layer thus replaces themetal wordlines discussed above. Because of this, there is no need formetal contact to connect the row wordlines R_(Y) to the N+ sourceregions 306. In general, this allows for higher density integration ofthe memory array.

For clarity purposes, it should be noted that the buried N+ layerforming the row wordlines R₁ and R₂ of FIG. 12 is shown, while the N+source regions 306 and the n+ drain regions 308 are not shown in the topview of FIG. 12.

FIG. 13 is a cross-section view of the silicon substrate taken alongline A-A′ of FIG. 12. The buried N+ layer 1301 is formed just underneaththe N+ source regions 306 and N+ drain regions 308. Indeed, the N+source regions 306 are in electrical contact with the buried N+ layer1301. Thus, the buried n+ layer 1301 replaces the metal row line R_(y)of FIG. 2. Additionally, the n+ drain regions 308 are also in contactwith the buried N+ layer 1301.

FIG. 14 shows a cross-section view of the substrate taken along ine B-B′of FIG. 12. In this embodiment, shallow trench isolations (“STI”) 1401serve to separate and isolate the memory cells. The buried n+ layer 1301is shown under the surface of the substrate, but still separated by theshallow trench isolations 1401.

The formation of the buried n+ layers 1301 will require additionalmasking and implantation steps. In one embodiment, or arsenic may beused as the dopent instead of phosphorous in order to limit thediffusion layer thickness in the deep submicron process. The buried n+layer 1301 can be formed using high energy ion implantation eitherbefore or after the formation of the thin gate oxide layer and/or thepolysilicon deposition. Alternatively, the buried n+ layer 1301 may bedeposited using an epitaxial deposition technique. Further, in order tobe CMOS logic process compatible, the lightly doped P-type implant isthe same as the logic NMOS threshold voltage V_(t) implant.

Compared to the embodiment shown in FIG. 2, the buried n+ layer 1301 canreduce the size of the memory array by 50% or more due to the smallercritical dimensions possible with the photolithography steps relative tometal and contact via formulation.

Finally, FIG. 15 shows the operation table for the embodiment of FIGS.12-14. The programming voltage V_(PP) is approximately 8-9 volts for agate oxide thickness of 32 angstroms or 5-6 volts for gate oxidethickness of 20 angstroms. Typically, the V_(DD) is the input/outputvoltage and is on the order of 3.3 volts or 2.5 volts. The power supplyvoltage of V_(CC) is typically 1.8 volts for 0.18 micron process or 1.2volts per 0.13 micron process. As you can see, there can be a range ofvoltages used in order to form the programming and reading functions.Note also that at lines 405 and 407 (for the unselected columns), thevoltage on the column bitline V_(bitline) is less than 0.5 volts. If theunselected column bitline is greater than 0.5 volts (in other wordsV_(t)), the previously programmed cells along the common column bitlinewill have a large leakage current through the programmed cells. Bylimiting V_(bitline) to under V_(t), this leakage current can be reducedor eliminated.

Various studies of oxide breakdown, which were performed in contextsdifferent than the memory cells 102 shown in the array 100, indicatesuitable voltage levels for breaking down ultra-thin gate oxides andestablishing that the breakdown is controllable. When an ultra-thin gateoxide is exposed to voltage-induced stress, breakdown in the gate oxideoccurs. Although the actual mechanisms leading to the intrinsicbreakdown of gate oxide are not well understood, the breakdown processis a progressive process passing through a soft breakdown (“SBD”) stagefollowed by a hard breakdown (“HBD”) stage. One cause of breakdown isbelieved to be oxide defect sites. These may act alone to causebreakdown, or may trap charges and thereby cause high local fields andcurrents and a positive feedback condition that leads to thermalrunaway. Improved fabrication processes resulting in fewer oxide defectsare reducing the occurrence of this type of breakdown. Another cause ofbreakdown is believed to be electron and hole trapping at various siteseven in defect-free oxide, which also leads to thermal runaway.

Rasras et al. performed a carrier separation experiment whichdemonstrated that, under positive gate bias, impact ionization of theelectrons in the substrate is the dominant source of the substrate holecurrent. Mahmoud Rasras, Ingrid De Wolf, Guido Groeseneken, RobinDegraeve, Herman e. Maes, Substrate Hole Current Origin after OxideBreakdown, IEDM 00-537, 2000. A constant voltage stress experiment wasperformed on ultra-thin oxide in an arrangement in which channelinversion was involved, and established that both SBD and HBD may beused for storing data, and that a desired degree of SBD or HBD may beobtained by controlling the time over which the gate oxide storageelement is stressed. FIG. 7 shows a schematic cross-sectionalrepresentation of the experimental setup. The effect of the constantvoltage stress on the ultra-thin gate oxide is shown in the graph ofFIG. 8, in which the x-axis is time in seconds and the y-axis is currentin amperes expressed logarithmically. FIG. 8 shows the gate andsubstrate hole current measured before and after soft and hard breakdownunder constant voltage stress. For roughly 12.5 seconds, the totalcurrent is substantially constant and dominated by an electron currentas measured by I_(g). The leakage is believed to be due toFowordlineer-Nordheim (“FN”) tunneling and stress-induced leakagecurrent (“SILC”). At about 12.5 seconds, a large jump in the measuredsubstrate hole current is observed, which signals the onset of a softbreakdown (“SBD”). The total current remains substantially constant atthis new level, albeit with some fluctuation in the substrate current,from about 12.5 seconds to about 19 seconds. At about 19 seconds, largejumps in both the electron current and the substrate hole current signalthe onset of hard breakdown (“HBD”). FIG. 8 shows that a desired degreeof SBD or HBD may be obtained by controlling the time over which thegate oxide storage element is stressed.

Sune et al. studied post SBD conduction in ultra-thin silicon dioxidefilms. Jordi Sune, Enrique Miranda, Post Soft Breakdown conduction inSiO2 Gate Oxides, IEDM 00-533, 2000. Various stages in thecurrent-voltage (“I-V”) characteristics of an ultra-thin gate oxide asdegradation proceeds are shown in FIG. 9, in which the x-axis is voltagein volts and the y-axis is current in amperes expressed logarithmically.FIG. 9 shows that a broad range of voltages may be used to program thegate oxide storage element, and that either SBD or HBD may be used tostore information in the gate oxide storage element. Several postbreakdown I-V characteristics are also included that show the evolutionfrom SBD to HBD. The amount of the leakage current resulting at SBD andHBD as well as at the intermediate situations between these two extremesis roughly linearly dependent on the magnitude of the voltage in a rangeof about 2.5 volts to 6 volts.

Wu et al. studied the voltage dependency of voltage acceleration forultra-thin oxides. E. Y. Wu et al., Voltage-DependentVoltage-Acceleration of Oxide Breakdown for Ultra-Thin Oxides, IEDM00-541, 2000. FIG. 10 is a graph of time-to-breakdown at 63%distribution vs. gate voltage in a semi-log scale measured n channelFETs (inversion) for oxide thickness varying from 2.3 nm to 5.0 nm. Thedistributions are in general agreement and are linear, furtherindicating that the process is controllable.

Miranda et al. measured the I-V characteristics of nMOSFET deviceshaving an oxide thickness of 3 nm and an area of 6.4×10⁻⁵ cm² after thedetection of successive breakdown events. Miranda et al., “AnalyticModeling of Leakage Current Through Multiple Breakdown Paths in SiO₂Films”, IEEE 39^(th) Annual International Reliability Physics Symposium,Orlando, Fla., 2001, pp 367-379. FIG. 11 shows the results correspondingto the linear regime in which “N” is the number of conducting channels.The results are quite linear, indicating that the path is essentiallyresistive.

In the embodiments described above, typically the n-type lightly dopeddrain (NLDD) implant is blocked to not have the gate overlap with thesource/drain N+ diffusions (see FIG. 3 and spacing D). This creates areverse diode between the wordline N+ S/D diffusion and the bitlinepolysilicon gate in the programmed cell. This results in reduced leakagebetween the un-selected wordline (biased at Vdd or even higher, in thecase where the floating wordline is charged up by a selected bitline atVpp through a programmed cell) to the unselected bitline (biased at 0 vor floating).

In the structures shown in FIGS. 1-6 and 12-15, the gate-oxide breakdownpoint is near the gate edge next to the wordline N+ diffusion region(see FIG. 5). The punch-through voltage from the wordline N+ diffusionregion to the oxide breakdown point is relatively low, so the reversediode is not effective to prevent the leakage current from unselectedwordlines to unselected bitlines. This may be undesirable for severalreasons.

Thus, in accordance with the present invention, the gate oxide that isproximal to the floating N+ diffusion region is made to be moresusceptible to breakdown than the gate oxide that is proximal to thewordline N+ diffusion region. While this can be done in a myriad ofways, two separate approaches are described herein: (1) making the gateoxide near the floating N+ diffusion regions thinner than the gate oxidenear the wordline N+ diffusion region (various methods for implementingwith two specific embodiments shown below); or (2) damaging the gateoxide near the floating N+ diffusion region with an implant so as tomake the gate oxide more susceptible to breakdown. It can be appreciatedthat the present invention is primarily directed to having a lowerbreakdown voltage at the floating N+ diffusion region and that anymanufacturing or structural implementation now possible or developed inthe future for achieving that aim is within the scope of this invention.

In one embodiment, as seen in FIG. 16, in order to shift the gate oxidebreakpoint away from the gate edge at the wordline N+ diffusion region,the gate oxide may be made thinner at the side of floating N+ source(diffusion) region (Alternative 1). The floating N+ diffusion regionconnects the adjacent two cells on the same wordline. Alternatively, thegate oxide may be made thicker at the side of the gate near the wordlineN+ region (Alternative 2). Note that the present invention can be easilyextended to a PMOS device where PMOS devices are formed inside anN-well.

Memory cells using this differential gate oxide MOS device have thefollowing advantages:

1. Cell programming by oxide breakdown always occurs preferentially onthe floating source side of the gate.

2. This provides a robust reverse diode between the drain (wordlinecontact) and the polysilicon gate (bitline) of a programmed cell.

3. The reverse diode punchthrough voltage is thereby greatly improvedcompared with the uniform gate oxide cell, where the gate oxidebreakdown could occur near the drain side (resulting in low punchthrough voltage of the reverse diode).

4. The programming voltage will be reduced (down to 3.5-5 V) since thesource side gate oxide thickness is much thinner than the standard gateoxide, which normally requires 6 to 6.5 V to program.

FIG. 17 illustrates the equivalent electrical circuit of thedifferential-oxide 1T memory cell. FIG. 18 shows the cell operation biasvoltages of one embodiment. There are several techniques that can beused to form such differential gate oxide, two of which are describedbelow.

Alternative 1: Using nitrogen (N2) implant (or other implant specieswhich can reduce the silicon oxidation rate) on one side of the gate tocreate differential gate oxide thickness. However, the differential gateoxide created by this method is not self-aligned.

As seen in FIGS. 19 and 20, after the Pwell and channel Vt implant, aphotomask is used to make a selected nitrogen implant in the siliconregion where the thinner gate oxide will be grown.

Next, turning to FIG. 21, the post implant photomask is removed, apreliminary gate oxidation cleaning step is performed, and a normal gateoxidation is performed to grow 20 A (for 0.13 μm generic process) on thenon-nitrogen implanted area. At the same time, a 10 to 15 A thinner gateoxide will be grown in the nitrogen implanted silicon region.

Turning to FIG. 22, after the gate polysilicon deposition andpolysilicon gate etch, this is followed by a NLDD implant and N+ S/Dimplant. Finally, a NMOS memory cell with differential gate oxidethickness on the source and drain are formed as shown.

Finally, turning to FIG. 23, after contact connection to the word linediffusions (wordline) and polysilicon bit lines (bitline), the memorycell with differential gate oxide NMOS devices are formed.

In an alternative method (Alternative 2), an isotropic etch is used onthe drain side followed by oxidation. The differential gate oxidecreated by this method is self-aligned. Specifically, as seen in FIG.24, after the gate polysilicon etch, a photoresist is used to cover thesource side, and an isotropic etch (usually wet etch) is performed tocreate an undercut on the drain (wordline) side gate oxide.

Next, as seen in FIG. 25, the photoresist is removed and an oxidationstep is performed to fill in the undercut gate, thus making a thickergate oxide on the drain (wordline) side. This is followed by regularNLDD implant, spacer deposition, spacer etch and S/D implant.

The above two methods describe making the thickness of the gate oxidedifferent based upon location relative to the source and drain of thetransistor. This is done in order to have a lower breakdown voltage forthe gate oxide nearer the floating N+ diffusion region. Another methodfor accomplishing the same task is to damage the gate oxide nearer thefloating N+ diffusion region by, for example, implantation of heavyions.

Specifically, another method is to implant heavy ions, such as As+, toselectively damage the gate oxide to make its oxide breakdown voltagelower than the normal gate oxide. This is also a self-aligned process.For example, as seen in FIG. 26, after the gate polysilicon etch, aphotoresist is used to cover the drain side. Then, an angled (15˜60degrees, and 2 way or 4 way rotation implants) As+ implant is performedon the floating source side. As seen in FIG. 27, the next step is toremove the photoresist and perform the regular LDD implant, spacerdeposition, spacer etch and S/D implant.

Note that the transistors used in the memory cells described herein inmost cases are normal low voltage logic transistors having, for example,an ultra-thin gate oxide thickness on the order of 50 Å for a 0.25 μmprocess, or on the order of 20 Å for a 0.13 μm process. The voltageacross such an ultra-thin gate oxide can be temporarily duringprogramming much higher than V_(CC), which typically is 2.5 volts for anintegrated circuit fabricated with a 0.25 μm process, and 1.2 volts foran integrated circuit fabricated with a 0.13 μm process. Such ultra-thinoxides typically can stand up to as much as 4 or 5 volts withoutsignificant degradation on the transistor performance.

The description of the invention and its applications as set forthherein is illustrative and is not intended to limit the scope of theinvention. Variations and modifications of the embodiments disclosedherein are possible, and practical alternatives to and equivalents ofthe various elements of the embodiments are known to those of ordinaryskill in the art. For example, the various voltages set forth in thevarious examples are only illustrative, since one has some discretion asto the precise voltage to select within a range of voltages, and thevoltages are in any event dependent on the device characteristics. Theterms row wordline and column bitline have been used to describe typesof lines commonly used in memories, but some memories may havealternatives thereto. Further, the various doping types may be reversed,such that an n-channel transistor described above may be replaced with ap-channel transistor. In such a situation, the p-channel transistorwould be formed in a large n-well and a buried p+ layer may be used.These and other variations and modifications of the embodimentsdisclosed herein may be made without departing from the scope and spiritof the invention.

1. A programmable memory cell useful in a memory array having columnbitlines and row wordlines, the memory cell comprising: a transistorhaving a gate, a gate dielectric between the gate and over a substrate,and first and second doped semiconductor regions formed in saidsubstrate adjacent said gate and in a spaced apart relationship todefine a channel region therebetween and under said gate; and whereinthe second doped semiconductor region of the transistor is connected toone of said row wordlines, and wherein said gate dielectric is formedsuch that the gate dielectric is more susceptible to breakdown near thefirst doped semiconductor region than said second doped semiconductorregion.
 2. The memory cell of claim 1 wherein said row wordlines areformed from a buried N+ layer.
 3. The memory cell of claim 1 wherein thegate dielectric of the transistor is thicker proximal to the seconddoped semiconductor region than to the first doped semiconductor region.4. The memory cell of claim 1 wherein the gate dielectric of thetransistor is damaged by ion implantation near said first dopedsemiconductor region.
 5. The memory cell of claim 1 wherein said gate isformed from one of said column bitlines.
 6. The memory cell of claim 1wherein said memory cells further including a programmed doped regionformed in said substrate in said channel region when said memory cellhas been programmed.
 7. A method of operating a programmable memoryarray comprising a plurality of row wordlines, a plurality of columnbitlines, and a plurality of memory cells at respective crosspoints ofthe row lines and column lines, said memory cells comprising atransistor having a gate, a gate dielectric between the gate and over asubstrate, and first and second doped semiconductor regions formed insaid substrate adjacent said gate and in a spaced apart relationship todefine a channel region therebetween and under said gate, the gate beingformed from one of said column bitlines, and the second dopedsemiconductor region of the transistor connected to one of said rowwordlines, said gate dielectric formed such that the gate dielectric ismore susceptible to breakdown near the first doped semiconductor regionthan said second doped semiconductor region, the method comprising:applying a first voltage to a selected one of the column bitlines andgate of a selected transistor; and applying a second voltage to aselected one of the row wordlines; wherein the first voltage and thesecond voltage form a potential difference across the gate dielectric ofsaid selected transistor to cause the formation of a programmed dopedregion in said substrate in said channel region of said selectedtransistor.
 8. The method of claim 7 wherein said selected transistor isread by applying a fourth voltage on the gate of said selectedtransistor and monitoring for a current flowing from said gate to saidselected column bitline.
 9. A programmable memory array comprising aplurality of row wordlines, a plurality of column bitlines, and aplurality of memory cells at respective crosspoints of the row wordlinesand column bitlines, each of the memory cells comprising: a transistorhaving a gate, a gate dielectric between the gate and over a substrate,and first and second doped semiconductor regions formed in saidsubstrate adjacent said gate and in a spaced apart relationship todefine a channel region therebetween and under said gate, the gate beingformed from one of said column bitlines; and wherein the second dopedsemiconductor region of the transistor is connected to one of said rowwordlines, said gate dielectric is formed such that the gate dielectricis more susceptible to breakdown near the first doped semiconductorregion than said second doped semiconductor region.
 10. The memory arrayof claim 9 wherein said row wordlines are formed from a buried N+ layer.11. The memory array of claim 9 wherein said column bitlines areconnected to said gate by a column bitline segment.
 12. The memory arrayof claim 9 wherein the gate dielectric of the transistor is thickerproximal to the second doped semiconductor region than to the firstdoped semiconductor region.
 13. The memory array of claim 9 wherein thegate dielectric of the transistor is damaged by ion implantation nearsaid first doped semiconductor region.
 14. The memory array of claim 9wherein said memory cells further including a programmed doped regionformed in said substrate in said channel region when said memory cellhas been programmed.